Change install include directory from tupl to lml namespace id

[willwray]

The current include directory usage sees tupl as a one-off standalone library.
The header install directory is tupl and that path is also required for usage.

#include <tupl/tupl.hpp>
#include <tupl/tupl_ties.hpp>
int main() {
    lml::tupl hello_world{43770, "world"};
    getie<1>(hello_world) = "World";
}

All tupl library headers have tupl_ prefix so these inlcudes are repetitive.

Initially there was no directory needed for usage; #include <tupl.hpp>
partly because the namespace id is configurable, for historical reasons,
so there was no obvious path name other than tupl.

Now there is a gathering legion of Lemuriad libraries all under one namespace 'lml'
chosen as the one true path (even if it does still remain configurable, for now).

tupl is a cornerstone component of the lml Universe. Make it so:

#include <lml/tupl.hpp>
#include <lml/tupl_ties.hpp>